fix: Sanitize owner label to comply with GCP label requirements

Fixes #6218

GCP labels only allow lowercase letters, numbers, underscores, and hyphens.
The owner flag (typically an email address) was being used directly as a
label value without sanitization, causing errors when it contained periods
or uppercase letters.

This change applies the existing _SafeLabelKeyOrValue() method to sanitize
the owner value before using it as a GCP label, ensuring compliance with
GCP's label requirements.

Author: jimmycgz

perfkitbenchmarker/benchmark_spec.py

@@ -1253,7 +1253,7 @@ def _GetResourceDict(self, time_format, timeout_minutes=None):
         'create_time_utc': now_utc.strftime(time_format),
         'benchmark': self.name,
         'perfkit_uuid': self.uuid,
-        'owner': FLAGS.owner,
+        'owner': self._SafeLabelKeyOrValue(FLAGS.owner),
         'benchmark_uid': self.uid,
     }